winit-core/ime: add more purposes and content hints

Also move IME example into `ime.rs` thus making IME integrations more
easily comprehendible.

Co-authored-by: Kirill Chibisov <contact@kchibisov.com>

winit/examples/x11_embed.rs

@@ -0,0 +1,71 @@
+//! A demonstration of embedding a winit window in an existing X11 application.
+use std::error::Error;
+
+#[cfg(x11_platform)]
+fn main() -> Result<(), Box<dyn Error>> {
+    use winit::application::ApplicationHandler;
+    use winit::event::WindowEvent;
+    use winit::event_loop::{ActiveEventLoop, EventLoop};
+    use winit::platform::x11::WindowAttributesX11;
+    use winit::window::{Window, WindowAttributes, WindowId};
+
+    #[path = "util/fill.rs"]
+    mod fill;
+
+    #[derive(Debug)]
+    pub struct XEmbedDemo {
+        parent_window_id: u32,
+        window: Option<Box<dyn Window>>,
+    }
+
+    impl ApplicationHandler for XEmbedDemo {
+        fn can_create_surfaces(&mut self, event_loop: &dyn ActiveEventLoop) {
+            let mut window_attributes = WindowAttributes::default()
+                .with_title("An embedded window!")
+                .with_surface_size(winit::dpi::LogicalSize::new(128.0, 128.0));
+            let x11_attrs =
+                WindowAttributesX11::default().with_embed_parent_window(self.parent_window_id);
+            window_attributes = window_attributes.with_platform_attributes(Box::new(x11_attrs));
+
+            self.window = Some(event_loop.create_window(window_attributes).unwrap());
+        }
+
+        fn window_event(
+            &mut self,
+            event_loop: &dyn ActiveEventLoop,
+            _window_id: WindowId,
+            event: WindowEvent,
+        ) {
+            let window = self.window.as_ref().unwrap();
+            match event {
+                WindowEvent::CloseRequested => event_loop.exit(),
+                WindowEvent::RedrawRequested => {
+                    window.pre_present_notify();
+                    fill::fill_window(window.as_ref());
+                },
+                _ => (),
+            }
+        }
+
+        fn about_to_wait(&mut self, _event_loop: &dyn ActiveEventLoop) {
+            self.window.as_ref().unwrap().request_redraw();
+        }
+    }
+
+    // First argument should be a 32-bit X11 window ID.
+    let parent_window_id = std::env::args()
+        .nth(1)
+        .ok_or("Expected a 32-bit X11 window ID as the first argument.")?
+        .parse::<u32>()?;
+
+    tracing_subscriber::fmt::init();
+    let event_loop = EventLoop::new()?;
+
+    Ok(event_loop.run_app(XEmbedDemo { parent_window_id, window: None })?)
+}
+
+#[cfg(not(x11_platform))]
+fn main() -> Result<(), Box<dyn Error>> {
+    println!("This example is only supported on X11 platforms.");
+    Ok(())
+}